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Doug, ASE Certified Technician
Category: Mitsubishi
Satisfied Customers: 8608
Experience:  Mitsubishi employed and Factory trained ASE certified technician
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I recently purchased a 2003 Mitsubishi Montero Sport. The

Customer Question

I recently purchased a 2003 Mitsubishi Montero Sport. The vehicle has 147000 miles, it is in great shape and ran great! I had the timing belt and water pump replaced because it was never changed. The vehicle ran good for awhile but the engine light came on. It eventually got worse and I wasn't able to drive the vehicle. I brought the vehicle back to the mechanic and he said the valve was bent in cylinder 6. My question is, did the mechanic install the timing belt wrong and that is why the valve bent. He said that a screw was found in the cylinder and that bent the valve.
Submitted: 1 year ago.
Category: Mitsubishi
Expert:  Roger replied 1 year ago.

Hello, I'd be happy to help! First, incorrect timing belt installation can definitely cause engine damage on this engine because it's known as an interference engine. If the timing belt was off when you took the car however you would have noticed it because it would run bad immediately even if it was off one tooth. Now if the tensioner was installed wrong it could have jumped a tooth after driving a while, which is possible and could cause what you have going on. There's no reason to remove spark plugs for a timing belt and if there was a screw in the combustion chamber it would have almost immediately caused problems, so that explanation you were given seems kind of sketchy in my opinion. Ultimately though, because we're not there we can't say for sure whether or not you're being lied to, but in my professional opinion something does not sound quite right. I would tow it to a different mechanic and have them check the timing belt for being in time correctly, that is something you can check with a little bit of tear down. If you have any more questions let me know. Please don't forget to rate, they won't compensate me unless I've been rated. Thank you!

Customer: replied 1 year ago.
Thanks. Do you think installing the tensioner caused the valve to bend or could something completely unrelated have caused the problem. Also, I did have to take the vehicle back to him after he installed timing belt because it sounded like a belt was squealing. Which, he did adjust but the noise came back.
Expert:  Roger replied 1 year ago.

A valve does not just bend, so something had to impact it to do that. If the tensioner was installed wrong or if the old one was not replaced it could have jumped time later and caused a valve to bend. A squealing belt would not be a timing belt that's the serpentine belts that you're referring to there. The thing that makes me lean towards a poor tensioner is the fact that it ran fine for a while then had problems later, that's just sounds like tensioner failure or incorrect tensioner installion.

Expert:  Doug replied 1 year ago.

I was just reading your situation and saw that Roger had opted out and thought I would offer some further assistance here.

First I would like to say I agree with everything Roger has indicated as well.

The allegation of there being a screw in the engine that cause the valve to bend is pretty improbable. While certainly it could cause such damage, as mentioned before there is no reason why a screw would get into the combustion chamber during the timing belt job. At most this could only happen if they were doing spark plugs as well (requiring removal of the intake) to allow a screw to drop into an intake port... however this would also require it to slide past the valve which is nearly impossible given both the incredibly tight clearance and the incredibly fast movement of the valve. It just would never make it into the chamber realistically unless it was a real tiny screw... in which case the odds of it bending a valve would be proportionally reduced greatly (it would just be heard as horrific noise while running).
And that is another reason this is so implausible of a scenario.... you had increasingly worsening condition of driving and warning light indication. If you somehow did manage to have a screw from a tune up job get dropped into the intake ports and somehow it manager to slip past the valve into the chamber, it would result in absolutely awful noise, like the engine was coming apart as it bounced around in there and then a sudden loss of compression if/when it bent the valve. And that would not cause the engine to no longer run. As evidence enough you can take one of these trucks and unplug a coil (killing two cylinders) and it will keep on running... and even half way decently. So losing compression on one cylinder only is not going to cause you to no start. Run poorly yes, but not a no start.

More likely is going to be that the tension was not set properly. This is a ongoing issue with independent shops that are unfamiliar with the tensioner set up getting these set wrong which leads to slipping of the belt. And when the belt slips it goes from running normally to running poorly to eventually not running at all as the belt slips beyond usable range, losing compression and bending valves. Knowing how common it is for these engines to get towed in from other shops that did exactly that, I would say it is more likely the case. There just isn't any reasonable explanation for how this screw got into the combustion chamber, let alone to cause the slow degrade of operation you experienced which is atypical of that scenario.